The present invention relates to cellular communications systems, in particular to cellular data communications systems, and the determination of cell borders in such systems.
Cellular communications systems such as GSM (Global System for Mobile Communications), AMPS (Advance Mobile Phone System) or D-AMPS (Digital Advance Mobile Phone System) are now well known. A characteristic of such cellular communications systems is that in the course of established communication sessions with mobile stations, for example mobile telephones, the mobile stations may move from one cell of the system to another cell so that the mobile station concerned communicates via different base stations, serving the different cells. Procedures and means for such xe2x80x9chandoverxe2x80x9d of a communication from one cell or base station to another are known in the context of these cellular communications systems.
An important factor for the successful operation of cellular communications systems is the design of the cells of hence the determination of the borders between adjacent cells. Cell design, and the determination of cell borders, are in general based upon measurements made in the field, xe2x80x9cby handxe2x80x9d, of actual communications conditions in the cells of the system. The measurements may be, for example C/I (carrier/interference) measurements. These measurements, and any consequent adjustments to cell design, represent a significant overhead and cost factor for the operator of a mobile communications system. This is particularly so because measurements of actual communications conditions have to be repeated, because the characteristics of the communications system and the cells within the system change with time. For instance, new cells may be added to the system or changes in environmental factors may affect the characteristics of cells.
Cellular communications systems, originally primarily concerned with voice communications, are now being developed to more efficiently carry data communications. One development, conceived for supplementing GSM communications systems, is the GPRS (General Packet Radio System) cellular data communications system, which is a packet based data communications system. GPRS, and other cellular data communications systems, also face the need to hand over an on-going communication from a mobile stationxe2x80x94in this case a mobile data terminal, such as a notebook computer or a multifunctional voice/data communications devicexe2x80x94from one cell or base station to another. GPRS, and other cellular data communications systems, also require careful cell design setting of cell borders. These aspects of the communications system may be more vital for data communications in order for data to be transmitted at necessary higher rates in comparison with rates used for voice communications, to establish a satisfactory level of data throughput in the system.
A feature which cellular communications systems may offer, and which is provided by the GPRS data communications system, is the availability of different coding schemes for data transmitted in the system and the ability to dynamically change from one coding scheme to another. This is known as dynamic link adaptation. Thus, for example, the GPRS system offers four different coding schemes (for instance for downlink, i.e. base station to mobile station, communications), giving different levels of error protection to transferred data and/or different levels of redundancy in transmitted data signals. In the GPRS system, for example, the coding scheme used, and hence the effective data transfer rate, can be dynamically adapted for an established communications link using link adaptation functionality provided in the system. This link adaptation functionality may use, for example, reports transmitted from mobile stations to the system in response to an order from the system. Such a report may for instance contain information relating to the last transmitted series of radio blocks from the system to the mobile station. Information included may be carrier strength (C), interference (I), signal variance (Sign_VAR), block error rate and bit error rate. Depending on this information, link adaptation functionality can select the coding scheme, of those available, for a next sequence of radio blocks. Hence, the effective date transfer rate can be changed depending for example on the current communications (e.g. radio) situation.
In a cellular communications system, in particular for data communications, the establishment of an efficient cell plan is a vitally important aspect of the system. The cell plan, i.e. the layout of cells, is a vital aspect of the system, especially for data communications. However, the effort and cost involved in establishing a cell plan, and adapting the cell plan as necessary to meet changed conditions, is a significant technical and economic burden on the system operator. If the cell plan is inappropriate to communication conditions generally applying to the areas covered by cells this can lead to acute inefficiencies in the system, so that the cost of monitoring the effectiveness of the cell plan, and adapting that plan, cannot be avoided.
In cellular communications systems, in particular for data communications, the provision of a satisfactory overall level of data throughput is also a matter of concern. Moreover, when a communications link has been established, if there is a change of data transfer rate in the linkxe2x80x94for example at cell handoverxe2x80x94the effects of this change of rate cannot always be predicted and may cause disruption of the data transfer.
According to one aspect of the present invention there is provided a method of determining a cell border between first and second cells in a cellular communications system in which communications with mobile stations can be effected at different effective transfer rates in dependence upon communication conditions with the mobile stations, and in which on-going communications with the mobile stations can be handed over from the first cell to the second cell, the method comprising
storing information relating to the transfer rates of communications with mobile stations prior to cell handover of those communications from the first cell to the second cell,
storing information relating to transfer rates of the communications with mobile stations subsequent to the cell handover of the respective communications concerned from the first cell to the second cell,
analyzing and processing the stored information, and in dependence thereupon determining the cell border between the first and the second cell such that, at communication handover, transfer rates before and after handover are more likely or most likely to be the same.
According to another aspect of the present invention there is provided apparatus for determining a cell border between first and second cells in a cellular communications system in which communications with mobile stations can be effected at different effective transfer rates in dependence upon communication conditions with the mobile stations, and in which on-going communications with the mobile stations can be handed over from the first cell to the second cell, the apparatus comprising
means for storing information relating to the transfer rates of communications with mobile stations prior to cell handover of those communications from the first cell (A) to the second cell (B),
means for storing information relating to transfer rates of the communications with mobile stations subsequent to the cell handover of the respective communications concerned from the first cell to the second cell,
means for analyzing and processing the stored information, and in dependence thereupon providing for determination of the cell border between the first and the second cell such that, at communication handover, transfer rates before and after handover are more likely or most likely to be the same.
Method and apparatus in accordance with the present invention can be particularly well employed in cellular data communications systems such as GPRS systems.
With method and apparatus in accordance with the present invention, when a cell border between first and second cells has been determined such that, at communication handover, transfer rates before and after handover are more likely to be the same than is the case with the presently set cell border (e.g. a more efficient cell border or cell plan has been determined because the presently set cell border does not provide that transfer rates before and after handover are most likely to be the same) actual adaptation of the cell border can be handled in a variety of different ways. For example, the system operator may be notified that a cell border change appears to be advisable but whether or not actual adaptation of cell border takes place may be left for the system operator to decide. Alternatively, the a cell border change may be effected automatically by changing parameters in base station controllers or other facilities in the system.
With method and apparatus in accordance with the present invention analysis of stored information, for determination of a more efficient cell border or cell plan, can be carried out at appropriate intervals. For example, the intervals may be set by the system operator, e.g. once every hour or once every day, or the intervals may vary in length for example such that analysis and determination takes place whenever enough information has been stored for useful statistical analysis.
The inventor has had the insight that if there is recorded a history of, for example, coding schemes selected by dynamic link adaptation at (i.e. immediately before and after) cell handover, this recorded history can be analysed and processed to reveal inefficiencies in the cell plan of a cellular communications system and to determine adjusted cell borders to reduce or minimize such inefficiencies.
The present invention offers a means of automatically determining or setting or adapting the cell plan of a cellular data communications system using information readily available from the system, without the need for xe2x80x9cby handxe2x80x9d measurement and without the need for costly or complicated additional features or equipment in the system. The present invention this provides that the effort and cost involved in determining and/or maintaining an appropriate cell plan, and in determining and/or effecting necessary adaptations to cell borders, can be significantly reduced.
The present invention can also provide for improvement in overall throughput in a communications system, which is of particular significance in the context of cellular data communications systems such as GPRS. Moreover, the present invention can reduce or minimize changes in data rates in on-going communications links at cell handover and thereby reduce the risk of disruption to data transfer.